Its probably as accurate an estimate as you can get when you're looking about 800 years in the past. Its pretty likely they got the general number range right on that one, considering the Mongols regularly depopulated entire cities in China, and were responsible for completely wiping out Khwarazmid Persia, one of the most prosperous empires in central Asia at the time. Then there's the casualties of their invasion of the Kievan Rus and the Middle East. I'm actually curious if these numbers figure in the Bubonic Plague, since the Mongols were responsible for introducing it to Europe.

I remember reading this ages ago, but this Chinese General, was severely outnumbered and the enemies were advancing. This General and his few troops were in a camp and he was told to shut the gates! But he kept them open, hid his army and all flags so the camp was in silence. The General sat on a ledge playing a flute (or something music wise). The enemies with a huge army, upon arrival grew weary and thought this was an ambush, and left. Later on the General launched a counter attack and won the battle lmao.